Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

A power management and distribution (PMAD) system includes a main DC bus,
an auxiliary DC bus, a power converter, one or more hybrid solid-state
circuit breakers, and a controller. The power converter converts power
from the main DC bus to provide a regulated output to the auxiliary bus.
Each hybrid SSCB includes a main switch connected between the main DC bus
and a load and an auxiliary switch connected between the auxiliary DC bus
and the load. The controller selectively turns the main switch and the
auxiliary switch On/Off to selectively supply power from the main DC bus
and/or the auxiliary DC bus to the load, and to selectively regulate the
voltage on the auxiliary DC bus.

Claims:

1. A power management and distribution (PMAD) system comprising: a main
direct current (DC) bus; an auxiliary DC bus; a power converter
configured to receive power from the main DC bus and to provide auxiliary
power to the auxiliary DC bus; one or more hybrid solid-state circuit
breakers (SSCBs) that includes a main switch connected between the main
DC bus and a load and an auxiliary switch connected between the auxiliary
DC bus and the load; and a controller configured to control the power
converter and the hybrid SSCB, wherein the controller is configured to
selectively turns the main switch and the auxiliary switch On/Off to
selectively supply power from the main DC bus and/or the auxiliary DC bus
to the load, and to selectively regulate the voltage on the auxiliary DC
bus.

2. The (PMAD) system of claim 1, wherein during a pre-charge operation
the controller turns Off the main switch, regulates the voltage provided
by the power converter to an initial voltage less than the voltage on the
main DC bus, and turns On the auxiliary switch.

3. The PMAD system of claim 2, wherein the controller gradually increases
the voltage provided by the power converter and turns On the main switch
when the voltage provided by the power converter reaches a threshold
level.

4. The PMAD system of claim 1, wherein during an overload operation the
controller monitors current supplied to the load, turns Off the main
switch and turns On the auxiliary switch in response to the monitored
current exceeding a first rated threshold.

5. The PMAD system of claim 4, wherein if the monitored current exceeds a
second rated threshold while the auxiliary switch is On, the controller
turns Off the auxiliary switch and reduces the voltage supplied by the
power converter to a minimum level, and wherein if the monitored current
falls below the first rated threshold, the controller turns On the main
switch.

6. The PMAD system of claim 5, wherein if the monitored current falls
below a third rated threshold while the main switch and auxiliary switch
are Off, then the controller increases the voltage supplied by the power
converter to the auxiliary DC bus.

7. The PMAD system of claim 1, wherein during a current limiting
operation the controller monitors current supplied to the load, turns Off
the main switch and turns On the auxiliary switch in response to the
monitored current exceeding a first rated threshold, and regulates the
voltage supplied by the power converter based on the monitored current.

8. A controller that manages the distribution of power to one or more
loads via a hybrid solid-state circuit breaker (SSCB) having at least a
main switch connected between a main direct current (DC) bus and a load,
and an auxiliary switch connected between an auxiliary DC bus and a load,
wherein a power converter receives power from the main DC bus and
provides a regulated output to the auxiliary DC bus, the controller
configured for: providing a first control signal connected to control the
state of the main switch , wherein the main switch distributes power from
the main DC bus to the load when On and prevents the distribution of
power from the main DC bus to the load when Off; providing a second
control signal connected to control the state of the auxiliary switch,
wherein the auxiliary switch distributes power from the auxiliary DC bus
to the load when On and prevents the distribution of power from the
auxiliary DC bus to the load when off; and providing a third control
signal that regulates the output provided by the power converter to the
auxiliary DC bus

9. The controller of claim 8, wherein the controller pre-charges
capacitive loads by turning Off the main switch via the first control
signal, regulating the auxiliary DC bus to an initial level via the third
control signal, and turning On the auxiliary switch to supply power to
the load via the auxiliary DC bus.

10. The controller of claim 9, wherein the controller gradually increases
the voltage on the auxiliary DC bus via the third control signal, and
turns On the main switch and turns Off the auxiliary switch when the
voltage on the auxiliary DC bus reaches a threshold level.

11. The controller of claim 9, wherein the controller provides overload
protection by turning Off the main switch and turning On the auxiliary
switch in response to a monitored current exceeding a first rated
threshold.

12. The controller of claim 11, wherein the controller turns Off the
auxiliary switch if the monitored current exceeds a second rated
threshold, and decreases voltage on the auxiliary DC bus, wherein the
second rated threshold is higher than the first rated threshold.

13. The controller of claim 12, wherein the controller turns On the main
switch and turns Off the auxiliary switch if the monitored current
decreases below the first rated threshold.

14. The controller of claim 9, wherein the controller provides current
limiting by turning Off the main switch and turning On the auxiliary
switch in response to a monitored current exceeding a first rated
threshold value, wherein the controller regulates the voltage on the
auxiliary DC bus based on the monitored current.

15. The controller of claim 14, wherein the controller reduces the
voltage on the auxiliary DC bus to a minimum level if the monitored
current exceeds the first rated threshold value for a pre-determined
period of time.

16. A hybrid solid-state circuit breaker (SSCB) comprising: a main switch
connected between a main DC bus and a load; and an auxiliary switch
connected between an auxiliary DC bus and the load, wherein the auxiliary
switch has a higher current rating that the main switch.

17. The hybrid SSCB of claim 21, wherein the main switch is a power
metal-oxide semiconductor field-effect transistor (MOSFET) and the
auxiliary switch is a thyristor.

18. The hybrid SSCB of claim 21, wherein during normal operation power is
supplied to the load via the main switch.

19. The hybrid SSCB of claim 21, wherein during pre-charge, overload, and
current limiting modes power is supplied to the load from the auxiliary
DC bus, wherein voltage on the auxiliary DC bus is regulated.

Description:

BACKGROUND

[0001] The present invention is related to power management and
distribution (PMAD) systems and in particular to PMAD systems employing
hybrid solid-state circuit breaker (SSCB) circuits.

[0002] Power management and distribution (PMAD) systems control the supply
of DC and/or AC power to various loads. Circuit breakers and switches are
commonly employed by these systems to not only control the supply of
power to the various loads but also to protect the load from fault
conditions. More recently, solid-state circuit breakers (SSCBs) have been
employed to provide fast response time, eliminate arcing during turn-off,
and prevent bouncing during turn-on transients. For example, power
semiconductor devices such as metal-oxide semiconductor field effect
transistors (MOSFETs) or insulated gate bipolar transistors (IGBT) are
commonly employed by SSCBs to control the distribution of power to a
load. In particular, power MOSFETs are commonly employed due to their low
conduction losses when conducting. The SSCB is part of a solid-state
power controller (SSPC) that includes a controller, current and voltage
sensors to monitor the current and voltage, respectively, provided to the
load by a power semiconductor device and emulates traditional circuit
breaker functionality by turning the power semiconductor device Off to
protect the associated load from fault conditions. Thermal sensors may
also be employed to protect the SSCB.

[0003] One of the specifics of SSCB is high overload requirements that may
exceed 1000% of the rated current. To achieve this high current rating, a
large number of MOSFET devices are connected in parallel. MOSFETS can be
easily paralleled, because of the positive thermal coefficient of their
on-state resistance. However, during overload transient conditions the
MOSFETs within SSCB may be subject to a current imbalance that results in
a particular device exceeding its peak current or continuous thermal
ratings. This condition creates reliability concerns. Unbalance may be
caused by parameter mismatches between semiconductor devices, gate drive
parameter mismatches, and/or power circuit parameter mismatches. There is
a need to improve SSCB and PMAD architecture to meet high overload
transient requirements without over sizing the SSCB.

SUMMARY

[0004] A power management and distribution (PMAD) system includes a main
DC bus, an auxiliary DC bus, a power converter, one or more hybrid
solid-state circuit breakers, and a controller. The power converter
converts power from the main DC bus to provide a regulated output to the
auxiliary bus. Each hybrid SSCB includes a main switch connected between
the main DC bus and a load and an auxiliary switch connected between the
auxiliary DC bus and the load. The controller selectively turns the main
switch and the auxiliary switch On/Off to selectively supply power from
the main DC bus and/or the auxiliary DC bus to the load, and to
selectively regulate the voltage on the auxiliary DC bus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 is a block diagram of a direct current (DC) power management
distribution (PMAD) system according to an embodiment of the present
invention.

[0006] FIG. 2 is a circuit diagram of a DC hybrid solid-state circuit
breaker (SSCB) according to an embodiment of the present invention.

[0007] FIG. 3 is a circuit diagram of a DC power converter according to an
embodiment of the present invention.

[0008] FIG. 4 is a flowchart illustrating functions performed by a PMAD
controller to provide pre-charge according to an embodiment of the
present invention.

[0009] FIG. 5 is a flowchart illustrating functions preferred by a PMAD
controller to provide pre-charge according to another embodiment of the
present invention.

[0010] FIG. 6 is a flowchart illustrating functions performed by a PMAD
controller to provide overload protection according to an embodiment of
the present invention.

[0011] FIG. 7 is a flowchart illustrating functions performed by a PMAD
controller to provide current limiting according to an embodiment of the
present invention.

DETAILED DESCRIPTION

[0012] FIG. 1 is a block diagram of direct current (DC) power management
and distribution (PMAD) system 10 according to an embodiment of the
present invention. Power distribution system 10 includes power converter
12, hybrid solid-state circuit breakers (SSCB) 14a, 14b, 14c, and 14d,
loads 16a, 16b, 16c, and 16d, and PMAD controller 17. Hybrid SSCB 14a,
14b, 14c, and 14d are selectively connected to distribute power to loads
16a, 16b, 16c, and 16d, respectively. Each hybrid SSCB 14a, 14b, 14c, and
14d is connected to receive power from main DC bus 18 and auxiliary DC
bus 20. In the embodiment shown in FIG. 1, auxiliary DC bus 20 is
provided by power converter 12, which receives power from main DC bus 18.

[0013] PMAD controller 17 generates control signals to control the
operation of power converter 12, and each of the plurality of hybrid
SSCBs 14a, 14b, 14c, and 14d. Control signal 22 is provided to power
converter 12 to enable and regulate the DC power supplied to auxiliary DC
bus 20. Control signals 24a, 24a' are provided to hybrid SSCB 14a to
control the supply of power from main DC bus 18 and auxiliary DC bus 20
to load 16a. In particular, control signal 24a is a main gate control
signal that controls the distribution of power from main DC bus 18 to
load 16a and control signal 24a' is an auxiliary gate control signal that
controls the distribution of power from auxiliary DC bus 20 Likewise,
control signals 24b, 24b', 24c, 24c', and 24d, 24d' are provided to
hybrid SSCBs 14b, 14c, and 14d, respectively, to selectively control the
supply of power from main DC bus 18 and auxiliary DC bus 20 to loads 16b,
16c, and 16d, respectively.

[0014] FIG. 2 is a circuit diagram of DC hybrid solid-state circuit
breaker (SSCB) 14a according to an embodiment of the present invention.
Hybrid SSCB 14a includes main switch device 26, auxiliary switch device
28, and diode 30. In the embodiment shown in FIG. 2, main switch 26 is a
power metal-oxide semiconductor field-effect transistor (MOSFET) device,
although in other embodiments other well-known solid-state switch devices
(SSSDs) may be employed. Main switch 26 is connected between main DC bus
18 and load 16a and is selectively turned On and Off via main gate signal
24a supplied by PMAD controller 17. When main switch 26 is On (i.e.,
"conductive"), then power from main DC bus 18 is supplied via main switch
26 to load 16a. When main switch 26 is Off (i.e., "non-conductive"), then
power from main DC bus 18 is prevented from being supplied via switch 26
to load 16a. In the embodiment shown in FIG. 2, a single main switch
device 26 is illustrated, although in other embodiments a plurality of
switching devices may be connected in parallel to satisfy desired power
loss requirements.

[0015] In the embodiment shown in FIG. 2, auxiliary switch device 28 is a
thyristor (SCR), although in other embodiments other well-known SSSDs may
be employed. Diode 30 is connected between the positive output provided
to a load and a return line. In the embodiment shown in FIG. 2 auxiliary
switch device 28 is a thyristor, selected for its high current rating as
composed with MOSFET devices. Auxiliary switch 28 is connected between
auxiliary DC bus 20 and load 16a and is selectively turned On and Off via
auxiliary gate control signal 24a'. When auxiliary switch 28 is On (i.e.,
"conductive"), power from auxiliary DC bus 20 is supplied to load 16a.
When auxiliary switch 28 is Off (i.e., "non-conductive"), then power from
auxiliary DC bus 20 is prevented from being supplied via auxiliary switch
28 to load 16a.

[0016] In general, during normal operation the gates of main switch 26 and
auxiliary switch 28 are On. However, the voltage provided by main DC bus
18 is typically higher than the voltage supplied by auxiliary DC bus 20,
and therefore the auxiliary SCR device is reversed bias, and power
supplied to load 16a is provided by main DC bus 18 via main switch 26.
Auxiliary DC bus 20 is utilized to implement various modes of operation
that require regulation of the input power supplied to one or more loads.
For example, PMAD controller 17 regulates auxiliary DC bus 20 during a
pre-charge mode, overload protection mode, or current limiting mode. PMAD
controller 17 regulates the power supplied to auxiliary DC bus 20 via
power converter 12 (shown in FIG. 1).

[0017] FIG. 3 is a circuit diagram of DC power converter 12 according to
an embodiment of the present invention, which includes regulator switch
32, capacitor 34, diode 36, inductor 38, and capacitor 39. In the
embodiment shown in FIG. 3, DC power converter 12 and associated
components are arranged in a non-isolated step-down (buck) converter
topology. In other embodiments, other DC-DC power converter topologies
may be employed to provide a regulated DC output on auxiliary DC bus 20
from main DC bus 18.

[0018] Regulator switch 32 is connected between main DC bus 18 and
auxiliary DC bus 20. PMAD controller 17 controls the state of regulator
switch 32 via control signal 22. When regulator switch 32 is turned On,
voltage supplied by main DC bus 18 is provided directly to auxiliary DC
bus 20, such that auxiliary DC bus 20 is unregulated. By varying the duty
ratio of regulator switch 32, the voltage supplied by main DC bus 18 is
stepped down to a lower voltage for supply to auxiliary DC bus 20. PMAD
system 10 (as shown in FIG. 1) utilizes power converter 12 and the
plurality of hybrid SSCBs 14a-14d to implement various modes of
operation, normal, pre-charge, overload protection, and current limiting.
Each mode of operation is discussed in brief, with FIGS. 4-7 providing
additional details on functions performed during each mode of operation.
In general, PMAD system 10 operates to provide power to loads 16a-16d via
the main switch (e.g., main switch 26 shown in FIG. 2). However, for
operations that require regulation of the bus power provided to hybrid
SSCBs 14a-14d, then PMAD controller 17 turns off the main switch and
turns on the auxiliary switch (e.g., auxiliary switch 28 shown in FIG.
2). PMAD controller 17 then regulates auxiliary DC bus 20 via regulator
switch 32 in power converter 12.

[0019] FIG. 4 is a flowchart illustrating functions performed by PMAD
controller 17 during a pre-charge operation according to an embodiment of
the present invention. The pre-charge mode is used with respect to
capacitive loads that may otherwise cause large in-rush currents at
start-up. Pre-charge is implemented by utilizing power supplied by
auxiliary DC bus 20 to charge the one or more capacitive loads, wherein
PMAD controller 17 regulates the voltage on auxiliary DC bus 20 to
prevent large in-rush currents. To illustrate, the functions are
described with respect to pre-charge of capacitive load 16a.

[0020] At step 40, PMAD controller 17 initiates a pre-charge operation.
This may be in response to a system start-up signal, or other input.
Typically, a pre-charge operation is initiated prior to a normal
operating mode.

[0021] At step 42, PMAD controller 17 turns Off main switch 26 (shown in
FIG. 2) of hybrid SSCB 14a via control signal 24a. Auxiliary switch 28 is
Off, and thus no power is supplied to load 16a.

[0022] At step 44, PMAD controller 17 regulates the voltage on auxiliary
DC bus 20 to an initial level. In the embodiment shown in FIG. 3, PMAD
controller 17 modulates control signal 22 to control the duty cycle of
regulator switch 32 (shown in FIG. 3) to regulate the output voltage of
power converter 12.

[0023] At step 46, PMAD controller 17 turns on auxiliary switch 28 via
control signal 24a'. As a result, power is supplied to load 16a from
auxiliary DC bus 20. By maintaining the voltage provided by power
converter 12 to auxiliary DC bus 20 at a relatively low voltage (as
described at step 44), the voltage (and therefore in-rush current)
supplied to load 16a remains limited and allows pre-charging of capacitor
load 16a.

[0024] At step 48, PMAD controller 17 increases the voltage on auxiliary
DC bus 20 by regulating the voltage supplied by power converter 12. As
discussed with respect to FIG. 3, PMAD controller 17 increases the
voltage supplied by power converter 12 by increasing the duty cycle of
regulator switch 32. In one embodiment, PMAD controller 17 increases the
voltage provided by power converter 12 at a pre-determined rate to
gradually increase the voltage supplied to the load without creating
large in-rush currents.

[0025] At step 50, PMAD controller 17 determines whether the voltage on
auxiliary DC bus 20 has reached a threshold or rated level. If the
voltage on auxiliary DC bus 20 has not reached the threshold level, then
the process continues at step 48, with PMAD controller 17 continually
increasing the voltage on auxiliary DC bus 20. If the voltage on
auxiliary DC bus 20 has reached the threshold level, then at step 52 PMAD
controller 17 turns on main switch 26 and turns off the gate of auxiliary
switch 28. As a result, power from main DC bus 18 is supplied to load
14a. At step 54, PMAD controller 54 provides a signal or otherwise
announces that pre-charge is complete. At step 56, operation in the
pre-charge mode is exited.

[0026] FIG. 5 is a flowchart illustrating functions performed by PMAD
controller 17 during a pre-charge operation of two or more loads
according to an embodiment of the present invention. In contrast with the
embodiment described with respect to FIG. 4, in the embodiment shown in
FIG. 5 two or more capacitive loads are charged via a pre-charge process.
To illustrate, the functions are described with respect to pre-charge of
respective loads 16a and 16b.

[0027] At step 62, PMAD controller 17 initiates a pre-charge operation.
This may be in response to a system start-up signal, or other input. At
step 64, PMAD controller 17 turns Off the main switch of hybrid SSCBs
14a-14d. For example, if loads 16a and 16b are capacitive loads to be
pre-charged, then PMAD controller 17 turns Off the main switches
associated with each hybrid SSCB via control signals 24a and 24b.
Auxiliary switches associated with hybrid SSCBs 14a and 14b are Off, and
thus no power is supplied to the load.

[0028] At step 66, PMAD controller 17 regulates the output voltage of
power converter 12 to a desired level for provision to auxiliary DC bus
20. In one embodiment, PMAD controller 17 regulates the duty cycle of
regulator switch 32 (shown in FIG. 3) to regulate the output voltage of
power converter 12.

[0029] At step 68, PMAD controller 17 turns on auxiliary switch 28
employed in a first hybrid SSCB associated with a first capacitive load.
For example, if load 16a is to be pre-charged first, then PMAD controller
17 turns on auxiliary switch 28 located in hybrid SSCB 14a via control
signal 24a', while maintaining other auxiliary SSCBs located in hybrid
SSCBs 14b-14d Off. As a result, power is supplied to load 16a from
auxiliary DC bus 20 at a voltage determined by the regulated output of by
power converter 12, while no power is supplied to loads 16b-16d. By
maintaining the voltage provided by power converter 12 at a relatively
low voltage (as described at step 66), the voltage (and therefore in-rush
current) supplied to load 16a remains limited.

[0030] At step 72, PMAD controller 17 increases the voltage on auxiliary
DC bus 20 by regulating the voltage supplied by power converter 12. As
discussed with respect to FIG. 3, PMAD controller 17 increases the
voltage supplied by power converter 12 by increasing the duty cycle of
regulator switch 32, controlled via control signal 22. In one embodiment,
PMAD controller 17 increases the voltage provided by power converter 12
at a pre-determined rate. As a result, the voltage and current supplied
to the load (e.g., load 16a) is increased gradually, thereby eliminating
in-rush currents.

[0031] At step 74, PMAD controller 17 determines whether the voltage on
auxiliary DC bus 20 has reached a threshold or rated level. If the
voltage on auxiliary DC bus 20 has not reached the threshold level, then
the process continues at step 70 with PMAD controller 17 increasing the
voltage on auxiliary DC bus 20. If the voltage on auxiliary DC bus 20 has
reached the threshold level, then at step 74 PMAD controller 17 turns On
main switch 26 and turns Off the gate of auxiliary switch 28 of the
hybrid SSCB (e.g., hybrid SSCB 14a) associated with the first capacitive
load (e.g., load 16a). As a result, power is supplied to the first
capacitive load from main DC bus 18.

[0032] At step 76, with the first capacitive load fully On and receiving
power from main DC bus 18, PMAD controller begins the process for turning
On the next or second capacitive load (e.g., load 16b). At step 76, PMAD
controller 17 reduces the voltage on auxiliary DC bus 18 to the initial
level discussed with respect to step 66. Once again, PMAD controller 17
regulates the voltage on auxiliary DC bus 18 via power converter 12.

[0033] At step 78, PMAD controller 17 turns On the auxiliary switch (not
shown) of hybrid SSCB 14b via control signal 24b'. In this way, the
second capacitive load begins receiving power from auxiliary DC bus 20,
but in-rush currents are prevented because the voltage on auxiliary DC
bus 20 is regulated to a relatively low level.

[0034] At step 80, PMAD controller 17 increases the voltage on auxiliary
DC bus 20. In one embodiment, PMAD controller 17 increases the voltage at
a pre-determined rate by regulating the output of power converter 12.

[0035] At step 82, PMAD controller 17 determines whether the voltage on
auxiliary DC bus 20 has reached a threshold or rated level. If the
voltage on auxiliary DC bus 20 has not reached the threshold level, then
the process continues at step 80 with PMAD controller 17 increasing the
voltage on auxiliary DC bus 20. If the voltage on auxiliary DC bus 20 has
reached the threshold level, then at step 84 PMAD controller 17 turns On
the main switch and turns Off the auxiliary switch (not shown) of hybrid
SSCB 14b associated with second capacitive load 16b. As a result, power
is supplied to the second capacitive load 16b from main DC bus 18.

[0036] At step 86, PMAD controller 17 turns On the auxiliary switch 28
associated with the first hybrid SSCB 14a and maintains in the On state
the auxiliary switch associated with second hybrid SSCB 14b. In this way,
the auxiliary switches associated with each hybrid SSCB, as well as the
main switches, are turned On as PMAD controller 17 transitions from
pre-charge mode to normal mode.

[0037] At step 88, PMAD controller 17 announces that pre-charge is
complete, and at step 90 exits the pre-charge mode. In this way, PMAD
controller provides sequential pre-charge of each capacitive load,
thereby avoiding in-rush currents for each capacitive load.

[0038] FIG. 6 is a flowchart illustrating functions performed by PMAD
controller 17 to initiate and provide overload protection according to an
embodiment of the present invention. To illustrate, the functions are
described with respect to an overload condition detected with respect to
hybrid SSCB 14a and respective load 16a. In general, with a detected
overload condition, PMAD controller 17 routes power through the auxiliary
switch of the hybrid SSCB, wherein the auxiliary switch is capable of
handling higher currents. If the overload condition persists, then the
auxiliary switch is turned Off and auxiliary DC bus 20 is de-energized
(i.e., voltage is reduced).

[0039] Overload protection is initiated at step 96. This does not mean
that PMAD controller 17 enters an overload protection mode, only that
PMAD controller 17 is beginning the process of monitoring for overload
conditions. In one embodiment, overload protection may be initiated when
PMAD controller 17 exits the pre-charge mode and begins normal operating
mode.

[0040] At step 98, PMAD controller 17 turns On main switch 26 of hybrid
SSCB 14a. If main switch 26 is already On, then PMAD controller 17
maintains main switch 26 in an On state. In addition, PMAD controller 17
turns on regulator switch 32 associated with power converter 12 to
provide an unregulated DC output to auxiliary DC bus 20. This represents
the "normal" operating mode for PMAD system 10.

[0041] At step 100, PMAD controller 17 determines whether the load current
exceeds a rated level. In one embodiment, PMAD controller 17 compares the
monitored load current to a trip threshold. In another embodiment PMAD
controller 17 utilizes a trip curve to determine the presence of an
overload condition. If the monitored current does not exceed a rated
level, then monitoring continues at step 100. If the monitored current
does exceed a rated level, then PMAD controller 17 initiates overload
protection mode at step 102.

[0042] At step 102, in response to the monitored current exceeding the
rated level, PMAD controller 17 turns off main switch 26 within hybrid
SSCB 14a to prevent power from main DC bus 18 being supplied to load 16a.
Auxiliary switch 28 remains On, and power is supplied to load 16a from
auxiliary DC bus 20. In one embodiment, auxiliary DC bus 20 is
unregulated at this point, and therefore provides a voltage very similar
to the voltage supplied by main DC bus 18.

[0043] At step 104, PMAD controller 17 determines whether the load current
exceeds a second rated (I 2t trip curve) threshold value. If at step 104,
PMAD controller 17 determines that the second rated threshold (or trip
curve) has not been exceeded, then at step 106 PMAD controller 17
determines whether the monitored load current still exceeds the first
rated threshold value utilized at step 100. If the monitored current does
exceed the first rated threshold value, then PMAD controller 17 continue
monitoring the current and comparing it to the second rated threshold
value at step 104. If the monitored current no longer exceeds the first
rated threshold value, then at step 108 PMAD controller 17 turns On main
switch 26 to supply power to load 16a from main DC bus 18. In addition,
PMAD controller 17 may turn Off auxiliary switch 28, although in other
embodiments auxiliary switch 28 may remain On, with a majority of the
current being supplied from main DC bus 18 via main switch 26. At step
110, with the monitored current now below both the first rated threshold
and the second rated threshold, the overload protection mode is exited
and the normal operating mode continues.

[0044] If at step 104, PMAD controller 17 determines that the monitored
current exceeds the second rated (I 2t) threshold, then at step 112 PMAD
controller 17 turns Off the gate of auxiliary switch 28. To prevent power
from being supplied to the load from either main DC bus 18 or auxiliary
DC bus 20, both main and auxiliary switches must be turn-off. When
implemented as a thyristor, auxiliary switch 28 can only be turn off by
reducing anode to cathode voltage so the current falls below the minimum
holding current level. Thus, PMAD controller 17 reduces the voltage on
auxiliary DC bus 20 to a minimum level (via power converter 12).

[0045] At step 114, PMAD controller 17 determines whether the load current
falls below a third rated threshold level. The third rated threshold
level employed at step 114 is less than the holding current level of the
thyristor (i.e., SCR). If the current has not decreased below the third
rated threshold level, then the process continues at step 112 with PMAD
controller 17 maintaining the gate of auxiliary switch 28 in the Off
position and reducing the auxiliary DC bus voltage 20 to a minimum level.

[0046] If at step 114 the monitored load current does fall below the third
rated threshold level, this indicates that the power has been
successfully disconnected from the faulty load. At step 116, PMAD
controller 17 turns on power converter 12 to supply power to auxiliary DC
bus 20. At step 118, PMAD controller 17 announces the presence of an
overload fault with respect to the faulty load, and exits the overload
protection mode at step 110.

[0047] FIG. 7 is a flowchart illustrating functions performed by PMAD
controller 17 to provide current limiting according to an embodiment of
the present invention. To illustrate, the functions are described with
respect to an overload condition detected with respect to hybrid SSCB 14a
and respective load 16a. In general, current limiting is provided in
response to an overcurrent condition, in which PMAD controller 17 turns
Off the main switch and limits the current by limiting the voltage made
available on auxiliary DC bus 20.

[0048] Current limiting mode of operation is enabled at step 122, although
current limiting will not be initiated unless a faulty load is detected.
Once again, operation is discussed with respect to hybrid SSCB 14a and
corresponding load 16a. At step 124, PMAD controller 17 turns On main
switch 26. In addition, PMAD controller 17 turns On regulator switch 32
associated with power converter 12, if not already On, to supply power to
auxiliary DC bus 20.

[0049] At step 126, PMAD controller 17 determines whether the load current
exceeds a first rated threshold, which may be the same or different than
the rated threshold level employed in the overload protection mode of
operation. If the monitored load current does not exceed the first rated
threshold, then PMAD controller 17 continues monitoring the load current
at step 126. If the monitored load current does exceed the rated
threshold level, then at step 128 PMAD controller 17 turns Off main
switch 26 and turns On (if not already On) auxiliary switch 28. In this
way, power is supplied to load 16a from auxiliary DC bus 20.

[0050] At step 130, PMAD controller 17 regulates the voltage on auxiliary
DC bus 20 via power converter 12 to maintain a desired current through
auxiliary switch 28. PMAD controller 17 selectively increases or
decreases the voltage supplied by power converter 12 to regulate the
current monitored through auxiliary switch 28.

[0051] At step 132, PMAD controller 17 determines whether the period of
operation of the current limiting mode has exceeded a predetermined time
limit. If the predetermined time limit has not been exceeded, then PMAD
controller 17 determines at step 134 whether the load current exceeds a
second rated threshold. In one embodiment, the second rated threshold is
the same as the first rated threshold employed at step 126, although in
other embodiments different thresholds could be employed. If the second
rated threshold is still exceeded, then the process continues at step 130
with PMAD controller 17 regulating the current through auxiliary switch
28 via current regulator 12.

[0052] If the second rated threshold level has been exceeded, then at step
136 PMAD controller 17 turns On main switch 26 and turns Off the gate of
auxiliary switch 28 to supply power to load 16a from main DC bus 18. That
is, PMAD controller 17 determines that the load fault has cleared and
normal operation can resume.

[0054] If at step 132, the pre-determined time limit threshold is
exceeded, then at step 142 PMAD controller 17 turns Off auxiliary switch
28. In addition, PMAD controller 17 reduces the voltage supplied by power
converter 12 to auxiliary DC bus 20 to allow the current through
auxiliary switch 28 to fall below the holding current (if implemented
with an thyristor), resulting in auxiliary switch 28 turning off and
disconnecting power to the faulty load 16a.

[0055] At step 144, PMAD controller 17 determines whether the monitored
load current has fallen below a third threshold level below the holding
current level of auxiliary switch 28, when implemented as a thyristor).
If the monitored load current has not decreased below the third
threshold, then at step 142 PMAD controller 17 continues maintaining
auxiliary switch 28 in the Off state and reducing the voltage made
available on auxiliary DC bus 20 to allow current through auxiliary
switch 28 to fall below the holding current level, resulting in auxiliary
switch 28 (when implemented as an SCR) turning Off and disconnecting
power to the faulty load 16a.

[0056] If the monitored current is less than the third rated threshold,
then an overcurrent condition is announced at step 146. Main switch 26
and auxiliary switch 28 remain Off to isolate faulty load 16a, but at
step 138 PMAD controller 17 controls power converter 12 to supply power
once again to auxiliary DC bus 20. At step 140 the current limiting mode
is exited.

[0057] While the invention has been described with reference to an
exemplary embodiment(s), it will be understood by those skilled in the
art that various changes may be made and equivalents may be substituted
for elements thereof without departing from the scope of the invention.
In addition, many modifications may be made to adapt a particular
situation or material to the teachings of the invention without departing
from the essential scope thereof. Therefore, it is intended that the
invention not be limited to the particular embodiment(s) disclosed, but
that the invention will include all embodiments falling within the scope
of the appended claims.